fix(go/genkit): use controlled flushing on JSON responses

[hugoaguirre]

json.NewEncoder() and encoder.Encode() were causing unexpected flushes to the response writer when streaming responses back to the reflection server causing invalid JSONS to be received during the stream execution.

Checklist (if applicable):

• PR title is following https://www.conventionalcommits.org/en/v1.0.0/
• Tested (manually, unit tested, etc.)
• Docs updated (updated docs or a docs bug required)